Problem: Which of these numbers is prime? ${21,\ 28,\ 45,\ 79,\ 93}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 21 are 1, 3, 7, and 21. The factors of 28 are 1, 2, 4, 7, 14, and 28. The factors of 45 are 1, 3, 5, 9, 15, and 45. The factors of 79 are 1 and 79. The factors of 93 are 1, 3, 31, and 93. Thus, 79 is a prime number.